1. Field of the Invention
The invention relates to wireless networking, and more particularly to a detection scheme of Orthogonal Frequency Division Multiplexing (OFDM) signals for an IEEE 802.11g receiver.
2. Description of the Related Art
With the emergence of a converged standard for wireless local area networks (WLANs), the stage is set for a multimode marketplace. Much like its wired predecessor, wireless Ethernet (802.11) will flourish in an environment characterized by multimode operation. Convergence of the separate 10- and 100-megabit per second technologies of wired Ethernet into the now familiar 10/100 networks accelerated the market's acceptance of wired Ethernet. The same should be expected of WLAN technology and the merging of the 802.11b and 802.11a versions of the standard into 802.11g.
In 1997, the first wireless Ethernet standard, known simply as 802.11, was adopted and published by the IEEE. This unified standard provided several modes of operation and data rates up to two megabits per second (Mbps). Work soon began on improving the performance of 802.11. The eventual results were two new but incompatible versions of the standard, 802.11b and 802.11a. The “b” version operated in the same frequency range as the original 802.11, the 2.4 GHz Industrial-Scientific-Medical (ISM) band, but the “a” version ventured into the 5 GHz Unlicensed National Information Infrastructure (U-NII) band. 802.11b mandated complementary code keying (CCK) for rates of 5.5 and 11 Mbps, and included as an option Packet Binary Convolutional Coding (PBCC) for throughput rates of 5.5 and 11 Mbps, and additional range performance. It also supported fallback date rates of 2 Mbps and 1 Mbps, using the same Barker coding used in the original 802.11 standard. 802.11a turned to another multi-carrier coding scheme, orthogonal Frequency Division Multiplexing (OFDM), and achieves data rates up to 54 Mbps. Because 802.11b equipment was simpler to develop and build, it arrived in the marketplace first. 802.11b technology soon established a foothold in the market and is proved the viability of WLAN technology in general.
In March of 2000, the IEEE 802.11 Working Group formed a study group to explore the feasibility of extending the 802.11b standard to data rates greater than 20 Mbps in the 2.4 GHz spectrum. For a year and a half, this group, which came to be known as the Task Group G, studied several technical alternatives until it finally adopted a hybrid solution that included the same OFDM coding and provided the same physical data rates of 802.11a. But this version of the draft standard, 802.11g, occupied the 2.4 GHz band of the original 802.11 standard. Several optional coding schemes were incorporated into 802.11g, including CCK-OFDM and PBCC, the latter of which provides alternative data rates of 22 and 33 Mbps. Briefly, the IEEE 802.11g draft standard requires the use of OFDM for data rates up to 54 Mbps and requires the support for CCK to ensure backward compatibility with existing 802.11b radios as mandatory parts. Because it integrates two technical solutions that had been totally separate and quite incompatible, the 802.11g standard thereby provides for true multimode operations.
Therefore, an 802.11g receiver must have the capability to detect both OFDM and CCK signals. In essence, the detection probability of a valid OFDM packet is required to exceed 90% within 4 μs when a receive level is equal to or greater than −82 dBm. The false-alarm probability, which means the probability of mistakenly detecting an OFDM packet as transmitting CCK packets, must be kept low enough to ensure a good packet error rate (PER) for the receiver's CCK module. Because Bluetooth devices and microwave ovens both operate in the same 2.4 GHz band, the 802.11g receiver also requires a low probability of false alarm with respect to Bluetooth and microwave oven radios in order to achieve high data throughput. In view of the above, what is needed is an efficient scheme of OFDM detection and CCK/Bluetooth rejection to meet the requirements.